The present invention is related to electronic solid-state fuses and more particularly to an electronic current limit circuit for limiting the current drain of a series connected load, and for switching the magnitude of the impedance of the current limit circuit from a low-impedance state to a high-impedance state to prevent excessive power dissipation by the circuit.
Basic current limiting devices such as fuses and resistive elements, for example, lamps, are well known in the art. However, fuses have the disadvantage in that they must be replaced each time that a current overload condition causes them to "open" circuit. Lamps suffer from the disadvantage of having high impedances and low speed response time with respect to momentary high energy, short duty cycle, pulses which may arise in solid-state circuits.
Other prior art circuits constituted by voltage regulators and thermal overload protection are also known. However these circuits normally "sense" an output voltage and have a feedback path which controls the supply voltage applied to the circuit. The disadvantage of the above circuits is that they provide voltage regulating techniques which require an additional current to provide the basic protection function which, is not necessarily the current that is seen by the load. Hence, more power is required by such circuits over that of the present invention.
Other solid state switching applications have utilized voltage-controlled devices of the breakdown or regenerative type. These devices are normally open voltage-controlled devices which are in a high-impedance state and then switched to a low-impedance state upon reaching a trigger or breakdown voltage brought about by positive feedback. However, the above devices do not sense the current seen by the circuit load and suffer from the same disadvantage of the previously described voltage regulators.
Therefore, a need exists to develop a solid state current limiting device for sensing the current through a series connected load element and to limit the current through the load when a predetermined magnitude of current is conducted therethrough because of circuit overload conditions.